Tuesday, September 25

A short history of my escarpment

I wander up a sandy trail to the base of the escarpment, and clamber up huge, dark, lava rocks to the tableland above. I jog through waving, yellow grasses and gaze at five small, dark volcanoes in a row on the western horizon.

One of the volcanoes, on the horizon. Cliff rocks in the foreground. Petroglyphs (chiseled by Pueblo Indians about 500 years ago) adorn the rocks. Photo courtesy of The Petroglyph National Monument.

How did the escarpment form? How did these friendly cones come into being?

About 30 million years ago, when camels and huge bears roamed New Mexico, a line of mantle swelled upwards, pushing against the Earth's crust. The crust buckled, and formed two immense fractures --- faults that ran the length of New Mexico from Colorado to Texas. A long splinter of crust dropped between the two faults, and the Rio Grande Rift was born.

Over the next 25 million years or so, the land stretched, creating tilted ranges and deep basins. Volcanoes erupted along the west side of the rift. Water drained out of Colorado Rockies into closed basins along the rift. Eventually, rivers and streams connected the basins to form the Rio Grande River so water flowed freely from the Rockies to the Gulf of Mexico.

The Rio Grande Rift continued to widen and filled with gravel and debris brought down from the mountains by the streams and rivers. A cycle of fill and cut ensued. The rivers dumped debris in the rift filling it, but also sliced through the fill, forming deep trenches.

The broad mesa I like to jog across is a gravel terrace filling the rift. The escarpment edge is the river's cut through the terrace.

The rift valley starts at the Rio Puerco and extends across the Rio Grande River to the Sandia Mountains. It's about 28 miles wide near Albuquerque.

The five volcanoes are, indeed, in a line --- one that parallels the rift. The oldest one erupted about 190,000 years ago and the last about 150,000 years. We think they're extinct now.

Basalt boulders. Photo courtesy of The Petroglyph National Monument.

Most of the volcanic (basalt) boulders I climb through shimmer in the sun a bronzy black. But those near the top of the escarpment show their true color: a light ash gray. Basalt rusts in the atmosphere, and turns dark brown or black.

By the way, the Sandia Mountains are not part of the Rockies. The Rocky Mountains were formed about 70 million years ago (about 40 million years before the Sandias) by a different process. Two shifting plates collided, eventually causing the uplift of land that built the Rockies.

Further Reading

Roadside Geology of New Mexico by Halka Chronic, 1987
Petroglyph Basalt

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